United Airlines says it's 'deeply' sorry for dragging off passenger amid revelations of his murky past

United Airlines has issued a new apology for what it has called a “truly horrific” stunt that saw a passenger ripped from an overbooked flight, as details of the man’s past are revealed.

United CEO Oscar Munoz said he was left outraged and “disturbed” over the forcible removal of 69-year-old passenger Dr David Dao by security officers which was filmed aboard the overbooked Flight 3411 from Chicago to Louisville.

“It’s never too late to do the right thing. I want you to know that we take full responsibility and we will make it right,” Mr Munoz said in a statement.

“No one should ever be mistreated this way.”

Mr Munoz promised a full review of policies and how such situations are handled in the future.

Meanwhile, several US media outlets have reported Dr Dao was previously convicted of obtaining drugs by fraud or deceit in 2004.

Dr David Dao and his wife and grandchildren. (Instagram) ()

Dr Dao had received sexual favours from a male patient in exchange for prescription medication, the New York Post reported, citing documents filed with the Kentucky Board of Medical Licensure.

Video of the incident showed Dr Dao being dragged down the aisle on his back by his hands, body limp, bleeding from the mouth, glasses askew and shirt pulled up above his navel.

The videos sparked outrage on social media - the second time in less than a month that United has been criticised for its treatment of passengers.

Mr Munoz was criticised for not apologising earlier and instead claiming the passenger had “defied” security orders.

Mr Munoz initially said the company could learn lessons from the situation, though he insisted he "emphatically" stood behind his employees.

"When we approached one of these passengers to explain apologetically that he was being denied boarding, he raised his voice and refused to comply with crew member instructions,” he said.

The airline said it had asked for volunteers to leave because additional flight crew needed to get to Louisville.

In late March, two teenage girls dressed in leggings were denied boarding on a United flight from Denver to Minneapolis because their form-fitting pants did not conform with the dress code for employees or family members using free passes.